Installing new linux distribution from an already running linux

Hi all,

Looks like there are no distributions that allow
installation from an already running linux ( different or older
distribution), except may be coLinux and WinLinux.

What I was looking for was some thing like load the CD/DVD, run
installer form the CD kind of installation.

Though it may be straight forward to have installers that runs from
other running system, seems it is not possible - except may be using
qemu etc. Is there any reason why there are no such installation
methods? Just wondering.